Utilizing the occurrence of plesiadapid mammals, we recognize both Tiffanian and Clarkforkian ages in the Fort Union Formation of the Piceance Creek Basin of Colorado. Five species of plesiadapids are described from eighteen fossil localities. The presence of Nannodectes gazini, Plesiadapis fodinatus and a new species Chiromyoides gigas indicate that at most eleven of the reported fossil localities are Tiffanian in age. The majority of the fossil specimens come from UCM locality 92177 (USGSD-2001) located 35.8 meters above the base of the Fort Union Formation, which we place in the Ti5 lineage zone. The occurrence of Plesiadapis dubius and Chiromyoides caesor in the Plateau Valley area suggests a younger Clarkforkian age for these stratigraphically higher localities. We are the first to recognize Tiffanian aged fossils in the Piceance Creek Basin, extending the mammalian record back to more than 58 million years ago.